Multivesicular liposomes are one of the three main types of liposomes, first made by Kim, et al. (Biochim, Biophys. Acta, 782:339-348, 1983), and are uniquely different from the unilamellar (Huang, Biochemistry, 8:344-352, 1969; Kim, et al. Biochim. Biophys. Acta, 646:1-10, 1981) and multilamellar (Bangham, et al., J. Mol, Bio., 13:238-252, 1965) liposomes in that there are multiple non-concentric aqueous chambers within. Previously described techniques for producing liposomes relate to the production of non-multivesicular liposomes; for example, U.S. Pat. Nos. 4,522,803--Lenk, 4,310,506--Baldeschwieler, 4,235,871--Papahadjopoulos, 4,224,179--4,078,052--Papahadjopoulos, 4,394,372--Taylor, 4,308,166--Marchetti, 4,485,054--Mezei, and 4,508,703--Redziniak. For a comprehensive review of various methods of liposome preparation, refer to Szoka, et al. (Ann. Rev. Biophys. Bioeng., 9:467-508, 1980).
Heterovesicular liposomes are lipid vesicles or liposomes with multiple internal aqueous chambers where at least two substances of different compositions are each encapsulated in separate chambers within one liposomes. The lipid vesicles or liposomes with multiple internal aqueous chambers include, but are not limited to, multilamellar liposomes, stable paucilamellar liposomes, and multivesicular liposomes. It is highly advantageous to provide a liposome delivery system in which two or more different substances are each encapsulated in separate compartments of a single liposome rather than encapsulated together in each compartment of the liposome.